Combustors are used in gas turbines for developing high pressure gases used in the generation of turbine power. In such turbine systems, gaseous reactant and fuel supplied by a compressor to a combustion chamber of the combustor are ignited and discharged into the inlet side of a turbine. The present practice is to use relatively refined fuels, such as kerosene or diesel fuels, or natural gas, that previously were relatively easily available; the gaseous reactant may be air, oxygen or oxygen enriched air, or carbon dioxide. By mixing and igniting the fuel and gaseous reactant, high volumetric heat release rates can be obtained under turbulent conditions by matching the concentrations and directions of fuel and gaseous reactant flow in a manner enabling high fuel concentration regions to overlap with regions of large shear stresses in the gaseous reactant flow, as disclosed in my British Pat. No. 1,099,959, issued Jan. 17, 1968.
It is recognized as desirable, especially in light of the energy shortage, to be able to use lower grade fuels, such as high nitrogen bearing, high aromatic content petroleum fuels, shale oils and coal liquids, for turbine power.
The major problems, in addition to efficiency and proper mixing of the gases and these fuels, are flame stabilization, elimination of pulsation and noise, and control of pollutant emissions, especially carbonaceous particulates and nitrogen oxides (NOx). Nitrogen oxides emitted from combustion processes have two main sources; namely, the fixation of atmospheric nitrogen from the combustion air at high temperatures, and the conversion of organically bound nitrogen compounds in the fuel to NOx. When the nitrogen content of the fuel exceeds 0.1% by weight, the fuel bound nitrogen plays an increasingly significant role in the emission of NOx. However, the laws governing formation of NOx from these two major sources are quite different. For example, the formation of NOx from atmospheric nitrogen is primarily dependent upon combustion temperature, and generally referred to as "thermal NOx"; whereas, the rate of formation of NOx from organically bound nitrogen in the fuel, generally referred to as "fuel NOx", is largely dependent upon local fuel-air mixture ratios and to a lesser extent upon temperature.
To minimize conversion of fuel bound nitrogen to NOx, it is necessary to first pyrolyse the fuel by heating it in an oxygen deficient environment, followed by admixing the combustion products and combustion air to complete the combustion process. Recent research has shown that given fuel rich conditions and sufficient residence time and temperature in the first or pyrolysis stage of the combustion process, fuel bound nitrogen may be rendered innocuous for NOx formation in the fuel lean second stage. This occurs through conversion to molecular nitrogen (N.sub.2) in the fuel rich first stage. However, care has to be taken when the rest of the combustion air is admixed to avoid locally high temperatures resulting in the formation of thermal NOx. This is achieved by admixing of combustion air and products of pyrolysis such that the temperature of the mixture is initially reduced by rapid mixing. This effects quenching of the reactions that would otherwise lead to the formation of thermal NOx. Downstream, a temperature rise occurs due to the up take of the oxygen by the pyrolysis products and exothermic combustion reactions. To effectuate these conditions, the temperature history of the mixture has to be closely controlled to insure that the combustion of soot and hydrocarbons may proceed to completion within the residence time in the combustor while maintaining temperatures in the lean stage below 1600.degree. K.
It is accordingly an object of the present invention to provide a combustor capable of minimizing the formation of nitrogen oxide products by tailoring the mixing and temperature history of the fuel according to known thermodynamic and chemical kinetic requirements of the combustion process.
Another object of the present invention is to provide a combustor comprising first and second combustion zones wherein a first fuel rich zone minimizes the conversion of fuel bound nitrogen to NOx and the second fuel lean zone fast mixes the combustion products from the first zone with combustion air at temperatures sufficiently low to prevent formation of thermal NOx.
Still another object of the present invention is to provide a combustor wherein cooling of the combustor walls is recuperative and capable of reducing heat loss in the fuel rich zone, without using any part of the gaseous reactant for film cooling.
Yet another object is to provide a combustor capable of achieving good control of the flow and mixing pattern while minimizing the pressure drop through the combustor.
Still a further object is to provide a combustor capable of maintaining temperatures sufficiently high for complete combustion without the formation of NOx products.